Decreasing the metallic content of liquids by an electrochemical technique

ABSTRACT

A method for decreasing the metallic content of a solution which comprises passing an electric current through a solution containing metallic material, which solution is contained as the electrolyte in a cell, said cell having at least one positive and one negative electrode, between which the current is passed, and wherein the electrolyte also contains a bed of particles, distributed therein, such that the porosity of the bed is from about 40 to 80%, porosity being defined as   The electrolysis of the electrolyte is continued until the desired reduction in the metallic content thereof is obtained.

United States Patent [1 91 Tarjanyi et al.

[ DECREASING THE METALLIC CONTENT OF LIQUIDS BY AN ELECTROCHEMICALTECHNIQUE [75] Inventors: Michael Tarjanyi, North Tonawanda; Murray P.Strier, Amherst, both of NY.

[73] Assignee: Hooker Chemical Corporation,

Niagara Falls, N.Y.

[22] Filed: Apr. 14, 1971 [21] Appl. No.: 133,923

[52] US. Cl 204/114, 204/149, 204/D1G. 3 [51] Int. Cl C02b l/82, C02c5/12 [58] Field of Search 204/86, 97, 91, 149, 204/152, 180 P, 130, 131,110, 106, 114, 52, 55, 105

[56] References Cited UNlTED STATES PATENTS 543,673 7/1895 Crawford204/110 X 698,292 4/1902 Kendall 204/1 10 3,457,152 7/1969 Maloney, Jr.et al 204/131 3,616,275 10/1971 Schneider 204/91 X 3,616,276 lO/l97lSchneider 204/91 X 3,616,356 lO/l971 Roy 204/152 FOREIGN PATENTS ORAPPLICATIONS 1,500,269 9/1967 France 204/DIG. 1O

1,584,158 12/1969 France 204/D1G. 10

OTHER PUBLICATIONS Le Goff et a1., Applications of Fluidized Beds inElec- 1 1 Aug. 28, 1973 trochem", lndust. & Engin. Chem, Vol. 61. No.10, October 1969, pp. 8-17.

Thangappan et a1., Copper Electrforming in Fluidized Bed," MetalFinishing, December 1971, pp. 43-49.

Primary Examiner-John I-l. Mack Assistant Examiner-A. C. PrescottA!torney--Peter F. Casella, Donald C. Studley, Richard P. Mueller andJames F. Mudd [57] ABSTRACT volume of particles volume of cell whereinthe 100 particles are distributed The electrolysis of the electrolyte iscontinued until the desired reduction in the metallic content thereof isobtained.

13 Claims, 1 Drawing Figure DECREASING THE METALLIC CONTENT OF LIQUIDSBY AN ELECTROCHEMICAL TECHNIQUE This invention relates to a process fortreating solutions which contain metallic materials and moreparticularly it relates to an improved electrochemical process fordecreasing the metalliccontent of a solution.

in various industries, solutions are utilized which contain metallicmaterials, and the disposal of these poses a significant pollutionproblem. For example, in the metal plating industries, the plating bathscontain copper, zinc and similar metals. Additionally, the effluent fromnumerous processes, such as chlor-alkali processes, frequently containmercury or lead/Although heretofore, various chemical techniques havebeen proposed for the treatment of such metallic containing effluents,these have generally been either inefficient or too expensive or haveresulted in the formation of products whose disposal presents as manypollution problems as the metallic materials themselves. Accordingly,there has recently been a great deal of effort expended in thedevelopment of new and different processes for the treatment of thesemetallic containing effluent solutions.

In Belgium patent 739,684, for example, there is described anelectrochemical technique wherein a semiconductive bed of solidparticles is used to oxidize various substances to non-toxic forms.Another process, utilizing an electrochemical technique, is described inNew Scientist June 26, 1969 Page 706. In these and similar processeswhich have recently been proposed, the electrochemical systems utilizedhave been found to be both inefficient, and/or uneconomical and requirefrequent changing of the bedof particles which is utilized. Accordingly,these systems have not met with any appreciable commercial utilization.

It is, therefore, an object of the present invention to provide animproved process for treating solutions containing metallic materials soas to reduce the metallic content of such solutions.

A further object of the present invention is to provide an improvedprocess for reducing the metallic content of a solution by means of anefficient and economical electrochemical treatment.

These and other objects will become apparent to those skilled in the artfrom the description of the invention which follows,

Pursuant to the above objects, the present invention includes a processfor treating a solution containing metallic materials to decrease themetallic content thereof which comprises passing an electric currentthrough the solution which contains the metallic materials, whichsolution is contained as the electrolyte in solutions may containvarying amounts of the metallic a cell, said cell having at least onepositive and one negative electrode, between which the current ispassed, and wherein the electrolyte also contains a bed of particles,distributed therein such that the porosity of the bed is from about 40to 80 percent, porosity defined as solutions containing metallicmaterials in this manner, it has been found to be possible to reduce theconcenvolume of particles volume of cell wherein the X100 particles aredistributed cadmium and the like, but also these metals in ionic form,such as Pb, Hg, Hg* and the like. These may be present as variouscompounds orcomplexes, both organic and inorganic. Additinally, since itis believed that the removal of the metallic materials from thesolutions treated by the present process involves reduction, thematerials going through various electrochemical reductions and resultingultimately in the metal itself which is deposited out at the cathode,the solutions treated may also contain various reduced states of themetallic materials as well.

The solutions containing metallic materials which are to be treated inaccordance with the present method may come from various sources. Thus,for example, they may be effluent streams from industrial plants whichhave relatively high concentrations of the metallic materials, as havebeen indicated heretofore. Additionally, however, the solutions treatedmay have a relatively low concentration of metallic materials, e.g. onepart per million or less, which solutions may come from municipal orother water treating plants. Thus, the method of the present inventionmay be used not only to reduce the relatively high content of metallicmaterials in industrial and similar waste streams, but, additionally,may also be used to effect substantially complete removal of relativelysmall amounts of metallic materials, as a final purification step in thetreatment of water intended for human consumption. The solutions treatedmay also contain various other components, in addition to the metallicmaterials and may include mixed effluent streams from several differentindustrial processes. Thus, for example, the solutions may contain, inaddition to the metallic materials, various chloride materials, such aschlorinated organics, chlorine, HCl, hypochlorites, hypochlorous acid,as well as, sulfates, fluorides, silicofluorides, phosphates, cyanides,and the like, as are typically present in plating bath and chlor-alkaliprocess effluents. Such solutions are, however, merely exemplary of theeffluent solutions which may be treated.

The pH of the solution to be treated may vary over a wide range, beingeither acidic, neutral or basic, pH values of from about 1 to 14 havingbeen found to be suitable. Desirably, where lead is the metal beingremoved, the pH is from about 4 to 7, with a pH of from about 6 to 13being preferred when the metal is mercury. Depending upon the makeup ofthe metalcontaining solution which is to be treated, adjustment of thepH may be done by the addition of various support electrolytes to themetallic solution. Suitable support electrolytes which may be used'areaqueous solutions of borates, ammonia, sodium chloride, sulfuric acid,calcium chloride, sodium cyanide, chloroacetates, sodium hydroxide,sodium bicarbonate, hydrochloric acid, and the like.

The temperature of the electrolyte, i.e., the solution being treated,may also vary over a wide range, the only criteria being that at thetemperature used, the electrolyte remain a liquid. Thus, temperatureswithin the range of about to 100 Centigrade have been found, generally,to be suitable. For economy in operation, however, it has frequentlybeen found to be preferred to utilize these solutions at ambienttemperatures. Similarly, the present process is desirably carried out atatmospheric pressure although either sub or super atmospheric pressuresmay be employed, if desired. It has been found in some instances,however, that the use of elevated temperatures, e.g., 6075C, may bedesirable in effecting a more rapid reduction in the metallic content,depending upon the particular support electrolyte, pH range, type andconcentration of metal which are used.

As has been noted hereinabove, the electrolyte, i.e., the solution beingtreated, is contained, during treat ment, in a suitable electrolyticcell and contains a bed of particles which are distributed in theelectrolyte in the cell, such that the porosity of the bed ranges fromabout 40 to 80 percent, porosity being defined as:

By determining the density of the particles used and weighing them, theterm volume of the particles in the above porosity formula may bereplaced by the value for the weight of the particles divided by thetrue density of the particles. The particle density can be measured byfilling a one liter container with particles, the weight of which isknown. Then, an electrolyte is added to the container to fill the voidsbetween the particles, the amount of electrolyte needed being measuredas it is added. The true density of the particles, in grams per cm", isthe weight of the particles in grams divided by the true volume of theparticles in cm. The true volume of the particles is the bulk volumeminus the volume of the voids in the particle bed, the latter being thevolume of the electrolyte which is added to the one liter container.Thus, the true volume of the particles in this instance would be 1,000cubic centimeters minus the volume of the voids, i.e., the volume ofelectrolyte added to the container.

It will, of course, be apparent that the porosity of the bed ofparticles maintained in the electrolyte which is being treated in thecell may be varied and that with different types of particles, under thesame operating conditions or with similar particles under differentoperating conditions, changes in the bed porosity will take place. Thus,the true density of the particle will vary depending upon the porosityof the particles themselves, e.g., graphite as compared to glass beads,with similar variations in density being effected by the electrolyteitself because of the differences in the surface tension of variouselectrolyte solutions. Additionally, since the particles of the bed aregenerally dispersed or distributed by the flow of the the electrolytethrough the cell,

volume of particles volume of cell wherein the X 100 particles aredistributed If the same quantity of particles were then distributed bythe flow of the electrolyte, such that the volume of the bed now reachedtwo liters, using its same formula, the porosity of the bed is nowvolume of particles in cc.

1000 cc. )Xloo v0lume of particles in cc.

Clearly, in the second instance, the porosity of the bed has, increased.As has been noted above, the porosity of the bed of particles dispersedin the electrolyte may range from about 40 to percent. In manyinstances, a preferred range for the bed porosity is from about 55 to 75percent with a specifically preferred range being from about 60 percentto 70 percent.

The particles employed to form the porous bed in the present processtypically are solid, particulate materials that may be conductive,non-conductive or semiconductive. By conductive it is meant that thematerial of which the particles are made will normally be considered anelectron-conducting material. Where they particles are conductive, theymay have a metallic surface, either by virtue of the particlesthemselves being metallic or by being made of non-conductive material onwhich a metallic surface has been deposited. Typical of the metals whichmay be employed are the metals of Group VIII of the Periodic Table, suchas mthenium and platinum, as well as other conductive elements, such asgraphite, copper, silver, zinc, and the like. Additionally, theconductive particles may be electrically conductive metal compounds,such as ferrophosphorus, the carbides, borides or nitrides of variousmetals such as tantalum, titanium, and zirconium, or they may be variouselectrically conductive metaloxides, such as lead dioxide, rutheniumdioxide, and the like. Where the particles are non-conductive, they maybe made of various materials, such as glass, Teflon coated glass,polystyrene spheres, sand, various plastic spheres and chips, and thelike. Exemplary of various semiconductive materials of which theparticles may be made are fly ash, oxidized ferrophos, zirconia,alumina, conductive glasses, and the like.

The particles used desirably range in size from about 5 to 5,000microns, with particle sizes of from about 50 to 2,000 microns beingpreferred. In many instances, a particularly preferred range of particlesizes has been found to be from about to 800 microns. Although it is notessential to the successful operation of the process of the presentinvention that all of the particles in the porous bed distributed in theelectrolyte have the same size, for the most preferred operation of theprocess, it has been found to be desirable if the range of particlesizes is maintained as small as is practical.

It has further been found that the density of the particles used shouldbe such, that in conjunction with the size and shape of the particles,it will provide the proper balance between the drag force created by theelectrolyte motion and the buoyancy and gravitational forces required toachieve particle dispersion or distribution at the desired bed porosity.Thus, where the particle dispersion is established against or inopposition to the buoyancy force, the particle densities typically mayrange from about 0.1 (less than the density of the electrolyte) to about1.0 grams per cc. Where the particle dispersion is achieved against orin opposition to the gravitational force, the particle densitiestypically may range from about 1.1 to grams per cc and preferably fromabout 1.5 to 3.5 grams per cc. The most preferred operating conditionshave been found to be when the particles are dispersed throughout theelectrolyte, within the cell, during the movement of the electrolyte andwhen the particles are more dense than the electrolyte. r

The electrolytic cell may be of any suitable material and configurationwhich will permit electrolysis of the metallic containing solution toeffect a reduction in its metal content and which will permit retentionof the porous bed of particles in the electrolyte, within the cell.Exemplary of suitable materials of construction which may be used forthe cell are various plastics, such as the polyacrylates,polymethacrylates, polytetrahaloethylenes, polypropylenes, and the like,rubber, as well as materials conventionally used in the construction ofchlor-alkali cells such as concretes. Additionally, the cells may bemade of metal, such as iron or steel. In such instances, electricallyinsulating coatings should be provided on the metal surfaces in the cellinterior or electrical insulation provided between the metal of the celland the electrodes.

The size of the electrolytic cell may also vary widely, depending uponthe nature andquantity of the metallic containing solution which is tobe treated. Thus, where appreciable quantities are involved, as in thetreatment I of industrial wastes or as a part of a water purificationsystem, the cell may be relatively large and include a multiplicity oftreating zones, whereas for the treatment of water for individual homeuse, appreciably smaller units may be utilized, similar in size toconventional soft-water treating units. Additionally, the cell may be ofa suitable size so as'to be portable, for use at camp sites, and thelike. Typically, the cell will have a suitable inlet and outlet meansfor introducing and removing the solution to be treated, means forretaining the porous bed of particles dispersed in the electrolytewithin the cell, means for supporting at least one positive and onenegative electrode in contact with the electrolyte in which the porousbed of particles is distributed and, if desired, a diaphragmdisposed-between the positive and negative electrodes.

The electrolytic cell has within it at least one positive and onenegative electrode. These are disposed within the cell so as to be incontact with the electrolyte in which is distributed the porous bed ofparticulate material. These electrodes may be formed of variousmaterials, as are known to those in the art. Typical of suitableelectrode materials which may be used are graphite; noble metals andtheir alloys, such as platnium, iridium, ruthenium dioxide, and thelike, both as such and as deposits on a base metal such as titanium,tantalum, and the like; conductive compounds such as lead dioxide,manganese dioxide, and the like; metals, such as cobalt, nickel, copper,tungsten bronzes, and the like; andrefractory metal compounds, such asthe nitrides and borides of tantalum, titanium, zirconium, and the like.

6 The positive and negative electrodes will be positioned withintheelectrolytic cell so as to be separated sufficiently to permit the flowof the electrolyte throughthe celland the movement of the particlewithin the electrolyte. It will be appreciated, of course,

that as the separation between the electrodes is increased the voltagenecessary to effect the desired reduction in the metallic impuritycontent of the electrolyte will also increase. Accordingly, .inm'anyinstances it has been found to be desirable if the separation betweenthe positive and negative electrode in the cell is from about 0.1 to 5.0centimeters, with a separation of from about 0.3 to about 3.0centimeters being preferred and a separation of from about 0.5 to 2.0centimeters being particularly preferred. Although particu lar referencehas been made to an electrolytic cell having one positive and onenegative electrode, it will be appreciated that the cell may be providedwith a plurality of electrode pairs, in much the same manner that such aplurality of electrodes are normally utilized in various commercial,large scale electrolytic continuous processes.

It will, of course, be appreciated that in addition to the amount ofelectrode separation, the flow of the electrolyte through the electrodearea will also be dependent upon the size and density of the particleswhich are distributed in the electrolyte to form the porous bed.Typically, this flow, which is described in terms of the linear flowvelocity of the electrolyte, will be within the range of from about 0.1to 1,000 centimeters per second. A preferred electrolyte flow velocityhas been found to be from about 0.5 to 100 centimeters per second with aflow velocity of fromabout l to 10 centimeters" per. second beingspecifically preferred. Under these operating conditions, currentdensities within the range of about 1.010 500 milliamps per squarecentimeter have been found to be typical of those which are utilized.

To further illustrate the present invention, reference is made to theaccompanying drawing which is a schematic diagram of a systemincorporating the electrolytic cell of the invention.

As shown in the drawing, this system includes an electrolytic cell (1)having a fluid inlet (6) and a fluid outlet (9). Within the cell (1) aredisposed a positive electrode(2) and a negative electrode (3). Althoughthese electrodes are shown as being separated by a diaphragm (4), inmany instances, the use of such a diaphram has not been found to benecessary. Where such a diaphragm is used, e-.g., to control theparticles in the anolyte or catholyte compartments, the diaphragm may beformed of various materials, such as-a Teflon coated screen, Fiberglas,asbestos,-porous ceramics and the like. .An electrolyte (8) is providedwithin the cell, which electrolyte is a solution containing metallicmaterial. A source (5) of this electrolyte is provided, from which theelectrolyte may be introduced into the cell through the inlet (6Distributed within the electrolyte (8) are; particles ,(7), whichparticles are distributed randomly through the electrolyte, the natureof the distribution depending upon the electrolyte flow, size and lytesources, cell inlets and outlets may be provided so that theintroduction of electrolyte into the anode and cathode compartments ofthe cell may be separately controlled. The cell is further provided withscreens (10) and (11), screen (11) serving to support the particles inthe cell and screen (10) serving to maintain the particles within thecell and prevent their discharge through the outlet (9). As the distancebetween the screens (10) and (11) is changed, the volume of that portionof the cell in which the particles are distributed will likewise vary,thus, varying the porosity of the bed of particles which is maintainedwithin the cell.

While it is not intended to restrict the operability of the presentinvention by any theory of operation, the use of particles in anelectrolytic cell in the manner which has been described, has been foundto have the following advantages. in a conventional electrolytic cell,such as a chlor-alkali cell, the amount of electrode surface at whichthe electrolytic reaction is conducted is dependent upon the surfacearea of the electrodes. Typically, this surface are will be about 1.3times l cm. With a typical cell volume of about 305 times cm, theresulting ratio of the electrode area per cell volume is about 0.037cmlcm". By the use of conductive particles in an electrolytic reaction,as in the process of the present invention, there is a significantincrease in the surface area at which the electrolytic reaction mayoccur. in Chemical and Process Engineering, February i968, page 93,there is described a cell containing an electrolyte having particlestherein. It was calculated that the electrolyte containing the particleshas an electrode area of about i [,500 cm and that the volume of thecell is about 153 cm. This gives a ratio of electrode area to cellvolume of about 75 cmlcm which, clearly, is significantly higher thanthat of an electrolytic cell having conventional electrodes.

Additionally, it is believed that by the use of the particles in theelectrochemical reaction, a mass transport phenomena may be takingplace. In this, the contact of metallic materials with the particles andelectrodes is dependent upon a number of variables, including theelectrolyte flow rate, the particles size, density and type, and theconcentration of the metallic material. From a consideration of all ofthe above variables, it has been found that the one condition which hasan effect upon all of them is the porosity of the bed of particles andthat this porosity, as defined hereinabove, is the determining factorthat makes possible a commercially feasible operation.

In order that those skilled in the art may better understand the presentinvention and the manner in which it may be practiced, the followingspecific examples are given. in these examples, unless otherwiseindicated, temperatures are in degrees centigrade and parts and percentare by weight. It is to be appreciated, however, that these examples aremerely exemplary of the present invention and the manner in which it maybe practiced and are not to be taken as a limitation thereof. I

in the following Examples, 1.5 liters of aqueous 0. lNCaCl,, 0.1N NaClor 0.1N HCl solutions, containing about 500 parts per million lead wereused. The solution was circulated through apparatus similar to thatshown in the drawing, having an electrode crosssectional area of 450 cm,for l5 minutes to allow for equilibration. A 50 milliliter sample wasthen with-- drawn and analyzed for pH and lead content. The analysesshowed substantially no reduction from theoriginal lead content of about500 parts per million, indicating little if any absorption on theparticles or electrodes in the cell. The solution was then electrolyzedunder the conditions indicated in the following table. The electrolytewas then drained from the apparatus and again analyzed for pH and leadcontent. All lead analy; ses were done by atomic absorption technique.In these Examples, there was no diaphragm used in the cell, theparticles were glass beads, having a particles size of 500 microns, theanode was graphite, the cathode was stainless steel and theseparationbetween the anode and cathode was 0.7 centimeters. Theelectrolyte flow rate was adjusted during the electrolysis so as to havea porosity of the bed of glass bead particles of 67 percent. The currentdensity used in all cases was 15 milliamps/cm'. Using this procedure,the following results were obtained:

peated using similar apparatus having an electrode cross-sectional areaof cm. From 700-800 milliliters of the electrolyte solution wascirculated through the cell. The cathode used was nickel, the anodegraphite and the separation between the electrodes was 0.4 cm. Theelectrolyte flow was adjusted so that the porosity of the bed of theglass bead particles was 65 percent. Using this procedure, the followingresults were obtained:

Time of Initial Final elec- Pb Pb Ex- Electrolyte Initial Finaltrolysls, content, content, ample solution p pH minutes p.p.m p.p.m.

5 0.1 N 0801: 5. 10 7.11 60 470 0. 2 6 0.1 N NaCh 5. 05 1. 68 570 9. 6 70.1N E01 1.20 0.93 180 5(1) 3.8

The procedure of Examples 5-7 was repeated with the exception that theelectrolyte used contained mercury, rather than lead. in Examples 8, 9and 10, the electrolyte solution was the filtrate obtained by filteringan industrial mercury containing waste effluent slurry through a coarseporosity sintered glass crucible. in the remaining Examples, theelectrolyte was obtained by mixing 50 grams of the slurry with 1 literof a 1.3N Na0Cl solution and filtering the resulting solution throughNo. 42 Whatman filter paper, the resulting filtrate being used as theelectrolyte. The solid resulting from the filtration of the originaleffluent slurry was found by X-ray analysis to contain Fe, Ca, K, S, andCl; minor amounts of Ba and Hg; and traces of Ni and Si. The filtratewhich was obtained was found to contain, in addition to Hg, Cl and K andtraces of Zn and S. Analysis of the filtrate for Hg was done by amodified Dow procedure using a Beckman Mercury Vapor Meter.

The conditions under which these solutions were electrolyzed were asfollows:

Example 8 Nickel anode; graphite cathode; 1.0 cm electrode separation;glass bead bed porosity 67 percent; current density 20 milliamps/cmExample 9 Same as Example 8 except the anode was platinum coatedtitanium and the current density was 50 milliamps/cm Example 10 Graphiteanode and cathode; 0.4 cm electrode separation glass bead bed porosity65 percent; current density milliamps/cm for first 240 minutes and 50milliamps/cm for last 60 minutes Example 11 Same as Example 10 exceptcurrent density was 50 milliamps/cm for first 120 minutes and 100milliamps/cm for last 120 minutes.

Example 12 Same as Example 10 except current density was 50 milliamps/cmfor first 60 minutes and 100 milliamps/cm for last 180 minutes and HClwas added to electrolyte to obtain indicated initial pH.

Using this procedure the following results were obtained:

Initial Hg content,

p.p.m.

Time 01 electrolysis, minutes Final Hg content, p.p.m.

I EXAMPLE 13 crons. The anode used was graphite, the cathode nickel, thearea of each electrode was 100 cm and the electrode separation was 1.35cm. After electrolysis for 52 minutes, at a current density of 15milliamps/cm and a voltage within the range of 2-3 volts, the Cu contentwas 5 ppm, the solution pH was 13.0 and the cyanide content was 05 ppm.Additionally, the cathode was found to have a characteristic coppercoating.

EXAMPLE 14 The procedure of Example 13 was repeated with the exceptionthat the solution treated was the effluent from a cyanide zincelectroplating bath having a pH of 12.54, a cyanide content of 200 ppmand a zinc ion content of 141 ppm. The graphite particles were of a sizeof 595-840 microns, the flow velocity was 2.0 cm/sec to produce a bedporosity of percent and the electrode separation was 0.4 cm. Afterelectrolysis for 1 10 minutes at 15 milliamps/cm and a voltage of 2-2.8volts, the pH was 12.8, the zinc ion content was 33 ppm and the cyanidecontent 0.5 ppm. Additionally, there was a characteristic zinc coatingon the cathode.

EXAMPLE 15 The procedure of Example l-4 was repeated with the exceptionthat 3.0 liters of a copper cyanide solution containing 1,353 ppm Cu and2,000 ppm CN was used. Periodically a 50 ml sample of the solution waswithdrawn and analyzed for copper using atomic absorption technique.Using this procedure, the following results were obtained:

Electrolysis Time Cut Concentration (Minutes) (ppm) pH Start 1353 12.860 559 12.57 93 12.50 21 12.40 11 12.35 1.8 12.25 210 1.5 12.15 240 1.012.20

The procedure of Examples l-4 was repeated using a zinc cyanide platingbath which had been diluted to 16,000 ppm CN, 11,260 ppm zinc and 0.44 NNa0H and a copper cyanide solution which had 16,000 ppm CN", 12,000 ppmcopper and 0.5 NKOH. These solutions were electrolyzcd using a currentdensity of 30 milliamps/cm and the following results were obtained:

While there have been described various embodiments of the invention,the compositions and methods described are not intended to be understoodas limiting the scope of the invention, as it is realized that changestherewithin are possible and it is further intended that each elementrecited in any of the following claims is intended to be understood asreferring to all equivalent elements for accomplishing substantially thesame result in substantially the same or equivalent manner, it

being intended to cover the invention broadly in whatever form itsprinciple may be utilized.

What is claimed is:

l. A method for decreasing the metallic content of a solution whichcomprises passing an electric current through a solution containingmetallic materials selected from mercury, lead cadmium and zinc, whichsolution is contained as tlie electrolyte in a cell, said cell having atleast one positive and one negative electrode between which the currentis passed, and wherein the electrolyte also contains a bed of dispersedparticles, distributed therein such that the porosity of the bed is fromabout 40 to 80 percent, porosity being defined as:

volume of particles cathodically reducing the metallic materials toelemental metal until the metallic content is reduced to a desirablelevel, plating the metal on the cathode, and removing said solution ofreduced metallic content from said cell.

2. The method as claimed in claim 1 wherein the electrolyte solution isan aqueous solution.

3. The method as claimed in claim 2 wherein the initial concentration ofthe metallic material in the electrolyte solution is from about 1 partper million to 10 percent by weight.

4. The method as claimed in claim 1 wherein the particles distributed inthe electrolyte solution have a density which is greater than that ofthe electrolyte.

5. The method as claimed in claim 1 wherein the particles distributed inthe electrolyte solution are conductive particles.

6. The method as claimed in claim 5 wherein the particles are graphite.

7. The method as claimed in claim 1 wherein the particles aredistributed within the electrolyte by flowing the electrolyte throughthe electrolytic cell in a direction opposed to the gravitationalforces.

8. The method as claimed in claim 7 wherein the electrolyte flowvelocity through the cell is from about 0.1 to 1,000 centimeters persecond.

9. The method as claimed in claim 1 wherein metal in the electrolyte islead and the electrolyte solution has a pH of from about 4 to 7.

10. The method as claimed in claim 1 wherein the metal in theelectrolyte is mercury and the electrolyte solution has a pH of fromabout 6 to 13.

11. The method as claimed in claim 1 wherein the porosity of the bed ofparticles is from about 55 to 75 percent.

12. The method as claimed in claim 11 wherein the porosity of the bed ofparticles is from about 60 to percent.

13. The method as claimed in claim 1 wherein the separation between thepositive and negative electrode within the cell is from about 0.1 to 5.0centimeters.

2. The method as claimed in claim 1 wherein the electrolyte solution isan aqueous solution.
 3. The method as claimed in claim 2 wherein theinitial concentration of the metallic material in the electrolytesolution is from about 1 part per million to 10 percent by weight. 4.The method as claimed in claim 1 wherein the particles distributed inthe electrolyte solution have a density which is greater than that ofthe electrolyte.
 5. The method as claimed in claim 1 wherein theparticles distributed in the electrolyte solution are conductiveparticles.
 6. The method as claimed in claim 5 wherein the particles aregraphite.
 7. The method as claimed in claim 1 wherein the particles aredistributed within the electrolyte by flowing the electrolyte throughthe electrolytic cell in a direction opposed to the gravitationalforces.
 8. The method as claimed in claim 7 wherein the electrolyte flowvelocity through the cell is from about 0.1 to 1,000 centimeters persecond.
 9. The method as claimed in claim 1 wherein metal in theelectrolyte is lead and the eLectrolyte solution has a pH of from about4 to
 7. 10. The method as claimed in claim 1 wherein the metal in theelectrolyte is mercury and the electrolyte solution has a pH of fromabout 6 to
 13. 11. The method as claimed in claim 1 wherein the porosityof the bed of particles is from about 55 to 75 percent.
 12. The methodas claimed in claim 11 wherein the porosity of the bed of particles isfrom about 60 to 70 percent.
 13. The method as claimed in claim 1wherein the separation between the positive and negative electrodewithin the cell is from about 0.1 to 5.0 centimeters.